The present invention relates to a polyimide film for use as a support for electrical circuit boards to which a metal layer is either laminated or deposited and to a process for the manufacture thereof. Specifically, it relates to a biaxially stretched polyimide film having excellent dynamic properties and in-plane isotropy, as well as improved dimensional stability, and a process for the manufacture thereof.
Due to its high heat resistance and good electrical insulation properties, polyimide film can be used as an electrical insulation material in a broad range of industrial applications which require heat resistance. More particularly, polyimide film can be used as a support for an electrical circuit board to which a metal layer is subsequently laminated or deposited and wherein an electrical part, such as an IC chip, can be soldered to the metal layer, thereby miniaturizing the electrical wiring. An electrical circuit board having a polyimide film support can also be folded to form a long continuous electrical circuit board to such an extent that polyimide film has come to occupy an important position as a support for electrical insulation applications. However, due to the diversification in the applications of electrical circuit boards and the increased density in the number of wirings, the polyimide film requires improvements in dynamic properties, in-plane isotropy, and dimensional stability for use as an electrical insulation support. In order to meet these requirements prior art proposals have been made based on improving physical properties of the polyimide film by stretching or by improving dimensional stability by the use of a copolyimide, or the like.
For example, Japanese Patent Application Publication Kokai 63-297029, discloses a process for improving the dynamic properties of an aromatic polyimide shaped article, comprising cyclizing and imidizing a solution of an aromatic polyamic acid in an organic polar solvent containing an imidization agent, shaping to give a self-supporting shaped article containing 20-87% by weight of volatile solvent and stretching the shaped article at a stretch ratio of at least 1.3 and heat-treating the stretched article at a temperature of at least 150.degree. C. The prior art process improves dimensional stability in the stretching direction, but it adversely affects not only the dimensional stability in the direction perpendicular to the stretching direction, but also the dynamic property of in-plane anisotropy.
Japanese Patent Application Publication Kokai 44-20878, discloses stretching a polyamic acid-imide gel film wherein the ratio of imide units to polyamic acid precursor units is at least 30:70 undirectionally at 20.degree.-550.degree. C. by at least 5%. However, the prior art reference fails to disclose how to reduce in-plane anisotropy of the film.
U.S. Pat. No. 3,619,461, issued on Nov. 9, 1971, discloses a process for preparing a polyimide film by immersing a film having a volatiles content of from 10 to 15% in a volatile liquid and heating and stretching the treated film to increase the orientation in at least one direction. In the invention process, the polyimide film is not immersed in a volatile liquid prior to stretching.
Japanese Patent Application Publication Kokai 64-16833, Kokai 64-16834, Kokai 1-131241 and Kokai 62-125329, and many similar proposals, disclose the use of special copolyimides for improving the dimensional stability of polyimide films. However, these disclosures fail to disclose a way to reduce in-plane anisotropy of the polyimide film.
The object of the present invention is to provide a polyimide film having improved dimensional stability and dynamic properties and excellent in-plane isotropy, and to a process for preparing said film.